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principal (adj.)

c. 1300, "main, principal, chief, dominant, most important;" also "great, large," from Old French principal "main, most important," of persons, "princely, high-ranking" (11c.), from Latin principalis "first in importance; original, primitive," from princeps (genitive principis) "first man, chief leader; ruler, sovereign," noun use of adjective meaning "that takes first," from primus "first" (see prime (adj.)) + root of capere "to take," from PIE root *kap- "to grasp." 

principal (n.)

c. 1300, "ruler, governor;" also "main part;" from principal (adj.) or from or influenced by noun uses in Old French and Latin. From mid-14c. in the sense of "money on which interest is paid;" 1827 as "person in charge of a public school," though meaning "head of a college or hall" was in English from mid-15c.